This invention relates to pyroelectric detector arrays for sensing electromagnetic radiation.
Radiation detectors can generally be divided into two broad categories, according to the nature of the energy detected. Quantum or photon detectors respond to discrete excitations caused by the action of individual photons. Thermal detectors, such as pyroelectric, bolometer, or thermoelectric junction devices, are sensitive to changes in the temperature of the detector material caused by the absorption of energy for incoming radiation. This absorption of the radiant energy may be direct, by adsorption within the detector material itself, or indirect, through absorption in some auxiliary structure which conducts the heat to the detector material.
Pyroelectric detectors utilize the spontaneous electrical polarization of a pyroelectric material. This polarization results from the anharmonic ionic vibrations which are possible in pyroelectric crystals. When the temperature of a pyroelectric material changes, the temperature change alters the spontaneous polarization of the material which, in turn, causes an electrical surface charge to flow. This surface charge can be measured and related to the intensity of the incoming radiation.
In order to achieve high sensitivity, the active element of a pyroelectric detector should have a small thermal mass, because a smaller mass will change temperature in response to absorbed radiation more quickly than a larger mass, leading to a faster and more sensitive detector. In addition, the active element of the detector should be thermally isolated from its surroundings to ensure a high temperature variation for a given amount of absorbed radiation. These two requirements, however, are difficult to satisfy simultaneously.
Minimizing the detector mass usually involves thinning the detector material, but thinner layers of pyroelectric material are more fragile and therefore require more substantial support structures. Additional support structures, however, conflict with the need for maximum thermal isolation, since a minimal amount of support is desirable for reduced heat transfer from the detector. Ideally, a pyroelectric detector would be totally isolated from its surroundings so that thermal losses from the detector would occur only by radiation. In practice, however, some provision for mounting the detector to a substrate is required, and heat diffusion into the substrate degrades the responsivity of the detector.
These design constraints are further complicated when multiple pyroelectric detectors are incorporated into a focal plan array, where the scene to be viewed is optically focussed on a two dimensional matrix of detector elements so that each detector images a particular portion of the scene. The array of detectors in interconnected to an integrated circuit chip (multiplexer) which provides signal gathering and processing functions. Each detector is connected to an input circuit on the multiplexer chip. The use of focal planes has become particularly desirable in the field of infrared imaging with the advent of improved signal processing techniques and photolithographic processes which make possible the fabrication of high density infrared systems employing a large number of detectors per unit area.
Although focal plane research has in the past concentrated on photovolatic detector designs, the need for cryogenic cooling for such detectors has led to the consideration of thermal detectors for use in medium performance applications. Thermal detectors do not require cooling and, as a consequence, are inherently simpler in design than photovoltaic detectors, resulting in weight savings, reduced power consumption, and lower costs. In addition, thermal detectors are uniformly sensitive over a wide range of the infrared spectrum and exhibit a nearly constant signal to noise ratio over a large frequency range. The problems involved in fabricating a pyroelectric detector with high thermal responsivity and isolation, however, are further exacerbated when an array of thin detectors must be fabricated and integrated with an interconnect structure exhibiting high thermal isolation.
Consequently, a pyroelectric detector array exhibiting superior thermal isolation for relatively thin detectors without sacrificing the physical integrity of the detector structure would be an important contribution to the art in the thermal imaging field.